Testimonial (part 3):

So I took those four drives and used them as sources for running Robocopy, to make seven new copies of my archive, onto seven newly purchased Toshiba canvio external hard drives. It was then that I discovered that three of the four were causing Robocopy to fail, so that I ended up with only one copy rather than seven. At that point, I saw that of the twelve good copies that I had thought that I had, only ONE was actually good enough to preserve my archive reliably.